The third component of complement, C3, plays an essential role in the defense of mammalian organisms against viral and microbial infections, by participating in both the classical and alternative pathways of the complement system. The cleavage of C3 by the classical or alternative pathway convertases liberates the C3 activation peptide, C3a, and yields C3b. C3b is unique in possessing multiple distinct functional sites for factors B, I and H, C5, P and the metastable binding site. In addition, the C3 fragments generated upon activation of the complement system interact with four different cell surface receptors and serve as modulators of complement dependent leukocyte functions. The elucidation of molecular features related to these complement functions will require structural analysis of C3 ligand interactions. The objective of the proposed project is to characterize the binding sites of C3 for C5, properdin and factors H and B. Our first objective will be to test, by an ELISA procedure, the ability of subdomain fragments of C3 to bind to C5, properdin, factors H or B. Then smaller C3 fragments containing the binding sites will be generated proteolytically or chemically. In case the binding sites prove to be composed of non-linear sequences, we propose to characterize the peptides containing the binding sites by cross-linking studies. The structure of the binding peptides will be determined by automated Edman degradation and their positions in the intact C3 molecule will be deduced by comparison of their sequences with the amino acid sequence of C3. Oligopeptides containing the binding site sequences will be synthesized and tested for their ability to bind to C3b and to inhibit the interaction of the different ligands with C3b. These peptides may have application as specific inhibitors of the complement system.